A secondary air supply system for an engine has been employed as part of the exhaust emission control, and has been used to supply air from an air cleaner to an exhaust port to cause uncombusted components remaining in an exhaust gas to burn again. The secondary air supply system is configured to include a secondary air control valve and a lead valve on a secondary air supply path that connects the air cleaner to the exhaust port. The secondary air control valve is under control of an engine control unit and is opened/closed according to the vehicle speed, the rotational speed of an engine, and the throttle position. In addition, the lead valve is configured to prevent the exhaust gas from flowing back into the air cleaner from the exhaust port, as well as to supply air from the air cleaner to the exhaust port (e.g., see Japanese Patent Laid-Open No. 2002-201935).
However, an air cleaner, a secondary air control valve, and a lead valve typically are arranged at complicated positions in conventional engines. Furthermore, the setting of secondary air supply paths, which connect an air cleaner, a secondary air control valve, and a lead valve with one another, is also complex. Thus, path lengths have been increased making it difficult to connect the secondary air supply paths to an air cleaner, a secondary air control valve, and a lead valve.